It's Frost Damage

To do this one guys it's not that hard and no-one has to die. All you need to understand is that the Evolved Vapor doesnt move when it casts Coalesce and the spell can be dodged by Line of Sight.

Choose 1 vapor that you will leave to evolve and tank it to the side of the ship away from the door to the central cabin. Kill other vapors etc as normal. Once the vapor starts it's cast EVERYONE retreat to the cabine, no boom, no knock-back. Get back outside, pick the next vapor and repeat.

At level 100, the damage is negligible, so all you have to do is stand there. Don't have to do any corner tricks or anything. However, now that it's entirely possible to one or two shot these bosses, you have to be careful when damaging Ripsnarl to get him to the vapor phase. Definitely do not use any high damaging abilities.

You don't have to kill the boss to get the achievement. What we did was stop attacking when Ripsnarl vanished and run inside the hut when the first add started casting coalesce. We died but got the achievement.